1. Field of the Invention
The present invention relates to a hydrogen-absorbing alloy, a fabrication method thereof, and an alkaline storage battery utilizing the hydrogen-absorbing alloy for its negative electrode. More particularly, a feature of the invention is an improvement in cycle life of an alkaline storage battery by enhancing its durability as well as maintaining a high hydrogen-absorbing capacity in a hydrogen-absorbing alloy used as its negative electrode.
2. Description of the Related Art
Conventionally, nickel-cadmium storage batteries have been commonly used as alkaline storage batteries. In recent years, nickel-metal hydride storage batteries using a hydrogen-absorbing alloy as a material for their negative electrodes have drawn considerable attention from the viewpoints that they have higher capacity than nickel-cadmium storage batteries and, being free of cadmium, they are more environmentally safe.
As the alkaline storage batteries of nickel-metal hydride storage batteries have been used in various portable devices, demands for further higher performance in the alkaline storage batteries have been increasing.
In the alkaline storage batteries, hydrogen-absorbing alloys such as a rare earth-nickel hydrogen-absorbing alloy having a CaCu5 crystal structure as its main phase and a Laves phase hydrogen-absorbing alloy containing Ti, Zr, V and Ni having an AB2 crystal structure as its main phase have been generally used for their negative electrodes.
However, these hydrogen-absorbing alloys generally do not necessarily have sufficient hydrogen-absorbing capability, and it has been difficult to increase the capacity of the alkaline storage batteries further.
In recent years, in order to improve the hydrogen-absorbing capability of the rare earth-nickel hydrogen-absorbing alloy, it has been proposed to use a hydrogen-absorbing alloy having a Ce2Ni7 type or the like crystal structure, rather than the CaCu5 type, by adding Mg or the like to the rare earth-nickel hydrogen-absorbing alloy (See, for example, JP-A 11-323469).
Further, it has been proposed to use a hydrogen-absorbing alloy having a specific composition wherein Mg or the like is added to the rare earth-nickel hydrogen-absorbing alloy for the purpose of increasing flatness characteristics of a phase and also reducing a pressure difference between hydrogen-absorbing pressure and hydrogen-desorbing pressure (See, for example, JP-A 2000-265229).
Particularly, in a case where the hydrogen-absorbing alloy having a hexagonal system structure or a rhombohedral system structure as its main phase and having a mol ratio of Mg being not less than 15% against the total mol of rare earth and Mg is used as a negative electrode of alkaline storage battery, because of its high equilibrium pressure, an operating voltage of the alkaline storage battery is improved.
Nevertheless, a problem in using the above-described hydrogen-absorbing alloy has been that charging/discharging causes the hydrogen-absorbing alloy to split easily. Consequently, the hydrogen-absorbing alloy reacts with an alkaline electrolyte, and oxidization of the hydrogen-absorbing alloy and drying out of the alkaline electrolyte are caused, so that a cycle life of the alkaline storage battery is deteriorated.